Adjustable lens systems for medical imaging or treatment applications are known, e.g. lens systems arranged for adjusting refraction of ultrasound. So-called fluid lenses or liquid lenses as described in WO 2003/069380 by Philips can be used for such applications. In essence, fluid lens technology allows light, or other rays or waves, to be refracted through alterations in physical boundaries of a fluid filled cavity with specific refractive indices. This process is known as electro-wetting, whereby the fluid within the cavity is moved by the application of a voltage across conductive electrodes, thus accomplishing a movement of the surface of the fluid. This change in surface topology allows waves to be refracted in such a way to alter the travel path to cause focusing or refracting to a desired location.
However, it is a well known problem with fluid type lenses, that rapid adjustment of the lens will cause the fluids to oscillate, thereby causing the refractive interface between the fluids to take uncontrollable shapes during the adjustment until the oscillations die out and the interface has changed to another stable shape. This means that real-time high quality imaging through such lens during the adjustment will be heavily blurred or distorted, and thus such lenses are in general not suited for recording of real-time images or video sequences. In order to have images of acceptable quality, the recorded image frame rate must not be higher than the lens can reach an equilibrium or stable state between two subsequent images. The same problems occurs for medical treatment applications of such lenses where precise control e.g. of ultrasonic ablation waves radiated towards living tissue is crucial. Thus, using known fluid lenses it is required that the ultrasonic ablation pulse rate is not higher than the lens can reach a stable state between two subsequent pulses.
WO 2006/030328 A1 describes an optical device with a fluid-to-fluid interface with a viscosity selected to provide a critical damping which allows fast switching between two pre-defined shapes of the interface. By selecting the fluids such that critical damping is obtained, oscillations in the interface can be avoided after the switching, thereby allowing the lens to be used quickly after switching has been performed. However, still WO 2006/030328 A1 does not address the issue of real-time recording of images through the optical device.